1. Field of the Invention
The present invention relates to a printed circuit board that reduces crosstalk noise generated between signal lines.
2. Description of the Related Art
A plurality of techniques are simultaneously being developed in order to realize electronic devices that have advanced capabilities. By one of such techniques, the transmission rate at which digital signals are transmitted in printed circuit boards incorporated into electronic devices is being increased. By another technique, the density of printed circuit boards incorporated into electronic devices is being increased.
When the transmission rate and the density are both improved at the same time, it is difficult to solve a problem related to crosstalk noise generated between signal lines. The factors related to the improvement of the transmission rate and the density in increasing crosstalk noise will be described. First, by increasing the transmission rate at which digital signals are transmitted, the rise time of the digital signals becomes short. The magnitude of crosstalk noise is proportional to the rate of change in the voltage in the waveform of a digital signal over time. Secondly, since the density is increased, the distances between signal lines become short, and therefore the magnitude of electromagnetic coupling between the signal lines becomes large. The magnitude of crosstalk noise is proportional to the magnitude of electromagnetic coupling between signal lines. As a result, an increase in the magnitude of crosstalk noise increases the possibility that a malfunction of a circuit is caused.
Now, the principle of the generation of crosstalk noise will be described. FIGS. 15A, 15B, and 15C are diagrams for explaining crosstalk noise generated in an existing printed circuit board. FIG. 15A is a diagram illustrating the structure of a circuit for explaining the principle of the generation of crosstalk noise. FIG. 15B is a graph illustrating the waveform of a signal and the waveforms of crosstalk noise. Signal transmitting units 203 and 204 are connected to one ends of a plurality of signal lines 201 and 202, respectively. Signal receiving units 205 and 206 are connected to another ends of the plurality of signal lines 201 and 202. A connection point between the signal line 201 and the signal receiving unit 205 is a receiving end 207, and a connection point between the signal line 202 and the signal receiving unit 206 is a receiving end 208.
The signal lines 201 and 202 that are arranged close to each other on a substrate each have a self-inductance Lo [H], and a coupled inductance Lm [H] exists between the signal lines 201 and 202. In addition, self-capacitances Co [F], which are capacitive components, exist between both the signal lines 201 and 202 and a ground layer (GND layer), and a coupled capacitance Cm [F], which is also a capacitive component, exists between the signal lines 201 and 202.
When a signal having an amplitude Vin [V] is output from the signal transmitting unit 203 with the rise time tr [s], the following theoretical formula of crosstalk noise observed at the receiving end 208 is obtained:−0.5×Vin×(Lo×Co)1/2×(Lm/Lo−Cm/Co)/tr 
The self-inductance Lo, the coupled inductance Lm, the self-capacitance Co, and the coupled capacitance Cm can be theoretically calculated from the structure of the printed circuit board such as the widths, the thicknesses, and the lengths of the signal lines 201 and 202, and the distances between the GND layer and both the signal lines 201 and 202.
The crosstalk noise generated at the receiving end 208 can be explained by dividing the phenomenon into two modes. One of the two modes is a differential mode, which is a state in which anti-phase crosstalk noise 401 having an amplitude whose sign is reversed from that of the amplitude of a signal 400 illustrated in FIG. 15B is generated. In this case, coupling between adjacent signal lines is considered to be inductive coupling, and Cm=0 in the expression (1). This mode can be expressed by a structure of a circuit coupled only by inductive components illustrated in FIG. 15A. The other mode is a common mode, which is a state in which crosstalk noise 402 having the same phase (the same sign of the amplitude) as the signal 400 illustrated in FIG. 15B is generated. In this case, coupling between adjacent signal lines is considered to be capacitive coupling, and Lm=0 in the expression (1). This mode can be expressed by a structure of a circuit coupled only by capacitive components illustrated in FIG. 15A.
In ordinary crosstalk noise, the crosstalk noises 401 and 402 are mixed. Because the inductive crosstalk noise 401 is dominant over the capacitive crosstalk noise 402 (Lm/Lo−Cm/Co>0) when a signal rises at the receiving end 208 in the printed circuit board, combined crosstalk noise 403 has an amplitude whose sign is reversed from that of the amplitude of the signal 400.
Crosstalk noise can be reduced by taking measures such as increasing the distance between adjacent signal lines or disposing the ground between signal lines, but the area of the printed circuit board is undesirably increased in these cases. In addition, as illustrated in FIG. 15C, in Japanese Unexamined Patent Application Publication No. 2001-257509, a capacitor 210 having a value Ca [F] of capacitance is connected between signal lines 201 and 202. At this time, a point of view on the characteristics for reducing the crosstalk noise is represented by the following expression (2):Lm/Lo=(Cm+Ca)/Co  (2)
However, in the above-described measures in an example of the related art, the effect of reducing the crosstalk noise is insufficient. Here, if ideal signal receiving units that have no parasitic capacitances are connected to signal lines, it can be theoretically confirmed that, by connecting a capacitor having a value Ca of capacitance that satisfies the expression (2) between the signal lines, the crosstalk noise can be sufficiently reduced. However, in practice, since signal receiving units that have parasitic capacitances are connected to signal lines, the parasitic capacitances of the signal receiving units also affect the magnitude of crosstalk noise. Therefore, even if the capacitor having the value Ca of capacitance that satisfies the expression (2) is connected, since parasitic capacitances of signal receiving units are not taken into consideration, the effect of reducing the crosstalk noise is small.